There are two main things I consider when judging these different curves. First off, it doesn’t really make sense for the score to get higher as a story gets older; you don’t see stories from a few months ago hopping back onto the front page for no reason. I’m assuming that the score must decrease as tt decreases and for a fixed vv. Mathematically, this is equivalent to saying that the partial derivative∂F∂t<0∂F∂t<0.